/Engine/Source/Runtime/RHI/Public/RHIResources.h
Returns the dimensions (i.e. the actual number of texels in each dimension) of the specified mip. ArraySize is ignored. The Z component will always be 1 for 2D/cube resources and will contain depth for volume textures. This differs fromGetSizeXYZ()which returns ArraySize in Z for 2D arrays.
FIntVector GetMipDimensions ( uint8 MipIndex ) const